Aerobic granular sludge has attracted extensive interest of researchers since the 90s due to the advantages of aerobic granules such as good settling ability, high biomass accumulation, being resistant to high loads and being less affected by toxic substances. Studies, however, which have mainly been carried out on synthetic wastewater, cannot fully evaluate the actual ability of aerobic granules.
Trang 1Study on aerobic granular sludge
formation in sequencing batch reactors for tapioca wastewater treatment
• Nguyen Thi Thanh Phuong
University of Technology, VNU-HCM
• Nguyen Van Phuoc
• Thieu Cam Anh
Institute for Environment and Resources, VNU-HCM
(Manuscript Received on January 21 st , 2013, Manuscript Revised May 04 th , 2013)
ABSTRACT:
Aerobic granular sludge has attracted
extensive interest of researchers since the
90s due to the advantages of aerobic
granules such as good settling ability, high
biomass accumulation, being resistant to
high loads and being less affected by toxic
substances Studies, however, which have
mainly been carried out on synthetic
wastewater, cannot fully evaluate the actual
ability of aerobic granules Study on aerobic
granular sludge was performed in
sequencing batch reactors, using seeding sludge taken from anaerobic sludge and tapioca wastewater as a substrates After 11 weeks of operation, the granules reached the stable diameter of 2- 3 mm at 3.7 kgCOD/m 3 day organic loading rate At high organic loads, in range of 1.6 - 5 kgCOD/m 3 day, granules could treat effectively COD, N, P with performance of
93 – 97%; 65 – 79% and 80 – 95%, respectively
Keywords: Aerobic granular sludge, sequencing batch reactor, tapioca wastewater
INTRODUCTION
Aerobic granular sludge formation and
applying them in practical wastewater treatment
was concerned for many years with some
advantages as follows: high Stability and
flexibility, Low energy requirements, Reduced
footprint, Good biomass retention, Reduced
investment and operational costs
Traditionally, flocculated sludge with low
settling velocities is applied and large settling
tanks are needed to separate clean effluent from
the organisms Besides large settling tanks,
separate tanks are needed to accommodate the different treatment processes Conventional processes need many steps for nitrogen, COD and phosphate removal, with large recycle flows and a high total hydraulic retention time Surplus sludge from a municipal wastewater plant needs different steps to dewater (e.g thickening and filterpressing) before it can be processed To overcome the disadvantages of a conventional wastewater treatment plant, biomass has to be
Trang 2grown in a compact form, like aerobic granular
sludge
The new aerobic granular sludge technology
has the ability to contribute to and improve the
biological treatment of wastewater Compared to
present wastewater treatment plants, similar
efficiencies at lower costs can be achieved with
the compact aerobic granular sludge technology
Granular sludge was first found in anaerobic
upflow anaerobic sludge blanket (UASB)
reactors to treat industrial wastewaters at the end
of the 1970s (Lettinga, 1980) [9] Anaerobic
granular sludge consists mainly of methanogenic,
syntrophic acetogenic and various
hydrolytical-fermentative bacteria and has been widely
applied in full-scale anaerobic reactors for
waste-water treatment since the 1980s (Hickey, 1991)
[6] Aerobic granular sludge is developed under
aerobic conditions and mainly used for the
aerobic degradation of organics and also for
nitrogen removal under aerobic and anoxic
conditions (Liu, 2004) [11] Aerobic granular
sludge was first reported in a continuous aerobic
upflow sludge blanket reactor by Mishima and
Nakamura (1991) [12] Aerobic granules with
diameters of 2 to 8 mm were developed, with
good settling properties Aerobic granulation has
since been reported in sequencing batch reactors
(SBRs) bymany researchers(Morgenroth et al.,
1997; Beun et al., 1999; Peng et al., 1999; Etterer
and Wilderer, 2001; Tay et al., 2001a; Liu and
Tay, 2002)and has been used in treating
high-strength wastewaters containing organics,
nitrogen and phosphorus, and toxic
substances(Jiang et al., 2002; Moy et al., 2002;
Tay et al., 2002e; Lin et al., 2003; Yang et al., in
press) Development of biogranules requires
aggregation of microorganisms This study
attempted to observe the biomass profile and
reactor performances for the treatment of COD,
N and P with the presence of successfully
developed aerobic granular sludge
PROCEDURES Experimental set-up
Experiments were performed in an open, cylindrical column typed SBR with a working volume of 5 L shown in Figure 1 Diameter, height of this model and working height are 90
mm, 1000 mm and 800 mm, respectively Influent was fed from a storage canister at a loading rate of 1.2 kgCOD/m3.day Aeration was provided by means of air bubble diffusers at a superficial air velocity of 5 L/min The reactor was operated in successive cycles of 3 h comprehended a feeding period of 5 minutes, a reaction period of 170 minutes, a settling period
of 2 minutes, an effluent withdrawal period of 3 minutes Granular development stage was operated in a time sequence of 5 minute filling,
170 minute aeration, 3 minute settling and 2 minute withdrawal The short settling time enhanced the granular development, enabled to select and retain good biomass, primarily granules which settling velocity is higher than 8 m/h
Wastewater and seed sludge preparations
Experiments were conducted with tapioca wastewater (after anaerobic tank) taken at cassava starch-processing plants in Binh Phuoc province (table 1) A suitable amount of nutrients were supplemented to ensure a feed COD:N:P ratio of 100:5:1 Prior to feeding the pH of the mixed liquor was adjusted to a level of between 6.8 and 7.2 using 1M NaHCO3 or 1M NaOH and 1M HCL
The initial seeding sludge was anaerobic sludge taken at Cassava starch processing factory
in Binh Phuoc province The initial MLSS and MLVSS concentration in the reactor were 7,273 mg/L, 4,500 mg/L, respectively And the ratio between MLVSS and MLSS was 62.3%
Trang 3Table 1 Characteristics of tapioca wastewater
taken at cassava starch- processing plants in Binh
Phuoc
Analytical methods
The diameter of granules was determined
using a microscope model Olympus BX 51 with
an attached DP 71 camera The sludge structure
and inner microbial organization were
characterized by Gram staining according to
Hucker and Conn methods The microbial
morphology was observed by using Olympus BX
51 microscope afterward Parameters such as
MLSS, MLVSS, COD, SVI, N-NO3-, N-NO2-,
Total Phosphorous, and alkalinity were carried
out according to Standard Methods [8]
Reactor operation
The experiment were carried out in two
stages: the first stage is sludge acclimation and
aggregation; the second one is granule maturation
and loading increasing The reactor was operated
in batch mode, feeding and withdrawal
automatically Each cycle had four steps: influent
filling, aeration, settling and effluent withdrawal
RESULTS AND DISCUSSION
Sludge acclimation and aggregation stage
After one week of acclimation, anaerobic
biogas sludge has transformed completely into
aerobic sludge, shown by the color of sludge
(switch from black to dark brown); MLSS
increased from 3,584 mg/L to 4,932 mg/L, while
the ratio of MLVSS / MLSS increased from
50.1% to 75% (Figure 2)
Figure 1 Experimental diagram
Figure 2 Change of MLVSS / MLSS ratio at the
organic loading rate of 1.2 kgCOD/m3.day
Figure 3 Change of SS, VSS in SBR corresponding to
different operation time
Trang 4
Figure 4 Change of COD at the OLR of 1.2 kg
COD/m3.day
Although biomass content reduced, COD
removal efficiency still increased from 70-81% to
a stable value of 91 - 93% at the end of the 2nd
week At the same time, the ratio of MLVSS /
MLSS increased to value of 79 % at the 14th day
Protozoa appeared in sludge such as Rotifer,
Cilia, and Flagella… At the 3rd week, the
settling time was maintained at the value of 3
minutes, biomass content decreased to 2754mg/L
as a result However, COD removal efficiency
was still higher than 92% (Figure 4) and the
sludge volume index (SVI) was lower than 50
mL/g due to a drop of water content in sludge
and an increase in biomass density It indicated
that aerobic granules were formed, which can
settle well and can treat the COD in wastewater
At the end of 3rd week, biomass content
increased to 3000 mg/L because aerobic granular
systems promote better biomass retention
compared to initial sludge, in addition, VSS
concentration of effluent was under 100mg/L
(Figure 3 and 4)
At this time, the sludge color switched from
dark brown to light brown, sludge flocs had a
tendency to segregate Granular core appeared in
streak shape, which had a diameter of 2mm
(Figure 5.e) Granules core was formed; the rate
of MLVSS / MLSS also increased rapidly and
reached the value over 80% at the end of the 3rd
week (Figure 2)
Development of granules (from the 4 th week onwards)
After 22 days of operation (the 4th week), granules began to appear and increase about both diameter and density afterward The sludge in the reactor was nearly completely granulized, and visually no suspended biomass was present Due
to the intensive mixing by aeration, the granular sludge became spherical with a smooth surface
At the 6th week, other forms of Rotifer and Cilia appeared at higher density, and Rotifer was still dominant (Figure 7.b, c) From week 7 to 9 (at the loading of 2.5kgCOD/m3.day), the microorganism as Protozoa, Rotifer, Cilia, Flagella in the granules gradually disappeared, bacteria were the majority of granules (Figure 7.d)
Aerobic granules diameter reached 2mm after
6 weeks (Figure 5.d) and was stable until the 13th
week Most of the biomass was kept in the reactor due to the good settle ability After the granules matured point, the granules were stable and dynamically balanced in the maturation phase In this phase, the granular size might still
be shifting mainly between 2.0 and 3.0 mm, but slowly and slightly, depending on the change of operational conditions And the mature granules contained Filamentous in core; the next layers were bacteria (mainly Gram negative), fungi, and protozoa (Figure 7.e) At week 11, density of bacteria in sludge was higher (Figure 7.f) From week 11, when the organic loading rate increased from 3.7 to 5kg COD/m3.day, granule diameter continued to increase to 3mm (Figure 5f) The change of granules diameter can be shown in figure 6 When the diameter increased, however,
it was difficult for the substances to diffuse into the granules core, leaded to broken granules if the OLR was increased As a result, the outside layer was taken out while the black core remained Subsequently, the broken granules recovered quickly, aggregated and increased the
Trang 5biomass The OLR was stopped at 5 kg
COD/m3.day to avoid breaking granules
At the organic loading rate of 2.5kg
COD/m3.day, VSS concentration increased to
6325 mg/L (at week 8), If the OLR increased to
3.7 - 5 kgCOD/m3.day, the value of VSS would
reach as high as 7360 mg/L at the loading of 5
kgCOD/m3.day (Figure 8) At OLR of 1.6
kgCOD/m3.day, SVI changed continuously in the
range of 38.4 - 39.6mL/g As OLR increased to
2.5 kgCOD/m3.day, granules were formed
developed stably leads to SVI decreased from
38.4mL/g at 6th week to 26mL/g at 9th week
When increasing the OLR up to 3.7
kgCOD/m3.day, granular sludge developed more
stably, tightly and heavily It can be proved
through SVI, SVI decreased from the 26mL/g at
9th week to 22.6mL/g at 11th week
When OLR increased to 5 kgCOD/m3.day,
more sludge can be formed and granules were
grown, as a result, SVI increased rapidly up to
64.69mL/g at 12th week and 65.61mL/g at 13th
week Research results about SVI variation with
different OLR were matched with the studies of
Bui Xuan Thanh, Nguyen Phuoc Dan [21] The
change of SVI at different loading was presented
in Fig 9
In this study, the removal performances of
COD, NH4, NO2-, NO3-, and total phosphorous
were investigated The results were shown in
Figure 10, 12, 13, 14 The following would
explain the removal performance At the
beginning, COD removal efficiency was 91.2%
When increasing OLR to 1.6 kgCOD/m3.day, 2.5
kgCOD/m3.day, 3.7 kgCOD/m3.day, and 5
kgCOD/m3.day COD removal efficiency was
93.2%, 95.6%, 94.8, and 95.1%, respectively
The COD removal efficiency was optimal at the
organic loading rate of 2.5 kgCOD/m3.day
(Figure 10) It reached the value of 95.6% while
MLVSS/MLSS ratio was over 90% Moreover,
MLVSS/MLSS ratio was always over 80% at all OLRs (Figure 11) These values were higher than using conventional activated sludge, which MLVSS/MLSS ratio was about 65 - 75% (Figure 11) The result also indicated that the biomass density was quite high in granule structure
Figure 5 Granules in different weeks (a initial
granules; b granule aggregation; c forming granules;
d growing granules; e stable granules; f granular
core)
Figure 7 Microorganism in the granules (a Rotife; b
Red Nematode; c Cilia; d protozoa around the granules; e granule structure; f bacilli and cocci
bacteria)
Trang 6
Figure 9 The variation of SVI at different organic
loading rates
Figure 10 COD removal efficiency at different
organic loading rates
Figure 11 The variation of MLVSS and MLVSS /
MLSS ratio at different organic loading rates
Figure 13 Change of NO2-, NO3- concentration at
different organic loading rates
Figure 14 Variation of N concentration at different
organic loading rates
Figure 12 Change of NH4 concentration at different
organic loading rates
Trang 7Phosphorous removal efficiency was
presented in Figure 15 Concentration of influent
phosphorous increased with increasing OLR
corresponding to COD At OLR of 2.5
kgCOD/m3.day corresponding to input P about
11mg / L, effluent P fell to less than 1.6mg/L,
effective treatment was about 80.0 - 95.2% At
higher loading of 3.7 - 5 kgCOD/m3.day
corresponding influent P in water were 18 and
23mg/L, respectively, effluent P was always less
than 4 mg/L Effective treatment was in range of
80.7-96.0% (Figure 15) The above results
indicated that P treatment in the model have been
rather stable P was removed by the synthesis of
the bacterial cytoplasm P was consumed rapidly
in the first minute of the aeration process The
higher OLR was operated corresponding to the
higher P concentration, the longer time consumed
the P content At OLR of 2.5 kgCOD/m3.day, P
was removed within 10 minutes of aeration
process, while at OLR of 3.7; 5 kgCOD/m3.day,
the time to treat P content up to 30 minutes In
the remaining time of the aeration process, P
concentration in the reaction tank was changed in
a range of 0.1 mg/L to 1 mg/L due to
decomposition and synthesis of bacterial cell in
reaction tank when the substrate was depleted
CONCLUSION
Aerobic sludge particles can be formed from the initial culture anaerobic sludge without carriers and with the short time for granulation formation (only in 3 adaption weeks) When the organic loading rate increased, the particle size of granules also increased and gained a stable size
of 2 - 3 mm at OLR of 3.7 – 5 kgCOD/m3.day After 6 weeks of operation, the granules were formed and grown with a range of 0.5 - 1.2mm Aerobic granules were in a good settling ability with SVI in the range of 22.6 - 64.6mL/g, much higher than conventional activated sludge with SVI > 100 mL/g [22] leads to decrease the settling time to 3 minutes Due to the accumulation of high level of biomass, granules can remove efficiently organic matter at high organic loading rate At OLR of 5 kgCOD/m3.day, with F/M = 0.79 - 1.63 (L/day), COD, nitrogen and phosphorus removal efficiency can reach 92-98%, 60-68% and 80-96%, respectively The study opens a new possibility for making granules and applications
of aerobic granules for high organic matter and nutrients pollution wastewater treatment in practice
Figure 15 Change of total phophorus concentration at different
organic loading rates
Trang 8Nghiên cứu tạo bùn hạt hiếu khí trên mô hình SBR để xử lý nước thải chế biến tinh bột khoai mì
• Nguyễn Thị Thanh Phượng
Trường Đại học Bách Khoa, ĐHQG-HCM
• Nguyễn Văn Phước
• Thiệu Cẩm Anh
Viện Môi trường và Tài nguyên, ĐHQG-HCM
TÓM TẮT:
Bùn hạt hiếu khí đã được rất nhiều nhà
nghiên cứu quan tâm từ những năm của thập
niên 90 do những ưu điểm của bùn hạt hiếu
khí mang lại như khả năng lắng tốt, tích lũy
sinh khối cao, chịu được tải trọng cao và ít bị
ảnh hưởng bởi các chất độc hại Tuy nhiên,
các nghiên cứu chủ yếu được tiến hành trên
nước thải tổng hợp nên chưa đánh giá được
đầy đủ khả năng xử lý thực tế của bùn hạt
hiếu khí Đề tài nghiên cứu tạo bùn hạt hiếu
khí trên nước thải thực tế là nước thải tinh
bột mì và qua đó đánh giá hiệu quả xử lý
chất hữu cơ của bùn hạt hiếu khí Trong nghiên cứu này, bùn hạt hiếu khí được nuôi cấy trên mô hình bể phản ứng từng mẻ (SBR) từ bùn nuôi cấy ban đầu là bùn kị khí Sau 11 tuần nuôi cấy, bùn hạt kích thước ổn định từ 2 – 3mm ở tải trọng 3.7 kgCOD/m 3 ngày.Với tải trọng hữu cơ cao, dao động từ 1.6 – 5 kgCOD/m 3 ngày, bùn hạt
xử lý hiệu quả COD, N, P với hiệu suất xử lý tương ứng đạt 93 – 97%; 65 – 79% và 80 – 95%
Từ khóa: bùn hạt hiếu khí, SBR, nước thải tinh bột mì
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